It is not allowed to change the signature of a Method. Even adding new optional Arguments to an existing Method would change the signature, so this is not allowed.

It is allowed to add metadata to the existing arguments of a Method, e.g. the EngineeringUnits Property (done with HasArgumentDescription References, see section 5 for an example). This does not include changing arguments from mandatory to optional or vice versa.

It is also allowed to change the description text of a Method parameter as long as this does not change the semantics of the argument.

If the signature of a Method would change, new OPC UA Clients might call old OPC UA Servers with the wrong signature (e.g. if an argument has become optional), or old OPC UA Clients may call the new OPC UA Server with the wrong arguments (e.g. if an argument has become mandatory). OPC UA Clients with prior knowledge of the Information Model do not necessarily read the Method arguments from the OPC UA Server, and even if they would, their implementation to call the Method would not work anymore.

If the intention is to change the signature of a Method, then a new Method with a new signature and a new BrowseName shall be defined instead. A common practice for the new name is to add a number to the existing name, i.e. the new Refresh Method is Refresh2.